1. Related Field
The present invention relates generally to the production of products from suspensions (e.g., as in tissuemaking, papermaking, boardmaking, and the like). More particularly, aspects relate to handling whitewater generated in a forming section.
2. Description of Related Art
Paper, tissue, board, and other products are often fabricated from a suspension (e.g., of cellulose in water). Such a suspension may be described as a stock. A forming section of a (e.g., paper or board) machine typically includes a headbox that injects stock between a loop of forming wire (e.g., a porous wire mesh or cloth) driven around a lead roll, and a loop of fabric (e.g., a felt or another forming wire), which is typically driven around a forming roll. Forces applied to the stock (e.g., via the headbox, the forming wire, the fabric, or the rolls) cause the water to pass through the forming wire, trapping the suspended material on the wire to form a web (e.g., of cellulose) between the forming wire and fabric. Water from the stock (so-called whitewater) is ejected through the forming wire. This “jet” or “spray” of whitewater is typically gathered and reused.
To reuse this spray of ejected whitewater, large amounts of entrained air should be removed from the water. Typically, the spray is decelerated and gathered to form a flowing stream of liquid water in a so-called “flume.” The flume typically comprises a relatively long channel (e.g., several meters or more) through which the water flows relatively slowly, so that air bubbles can rise to the surface prior to reuse of the water. The flume is typically several meters (even tens of meters) long and over a meter (even several meters) wide. Because flow through the flume should be slow (allowing air bubbles to rise to the surface), the flume typically has a very shallow slope away from the forming section toward the fan pumps used to recycle the flume water. Such long flume lengths are typically necessary to maximize the tendency of air bubbles to rise up out of the flume water. It may be desirable to shorten flume length (while still removing undesired air), and so it may be advantageous to remove air (e.g., from the ejected whitewater) as quickly and efficiently as possible.
The reuse of water (e.g., to make additional stock with added suspended material) may be improved with more efficient air removal from the water. It is desirable to produce an efficient de-aeration process (e.g., to reduce complexity, cost, and/or energy consumption) associated with recycling the whitewater.
EP 1 424 437 A1 describes collecting drainage water from a forming roll in a twin-wire former of a paper machine. U.S. Pat. No. 4,714,522 discloses a system in which whitewater is caught in a whitewater trough which is provided with deflection vanes. U.S. Pat. No. 4,028,174 discloses a curved deflector for intercepting high velocity sprays of liquids. U.S. Pat. No. 6,096,120 discloses a double acting deaeration vessel. PCT patent application no. PCT/IT2007/000600 describes a wet forming paper machine with systems to reduce turbulence.
U.S. Pat. No. 8,784,538 describes a solution in which a first chamber part comprises a guide wall portion for redirecting drainage water in a predetermined second flow direction that differs from the first flow direction, said guide wall portion being formed by a plurality of curved guide walls which define a plurality of curved and substantially parallel flow channels for the drainage water and that the guide walls are arranged to interact with the drainage water in such a way that the drainage water is decelerated and air is forced out of the drainage water; and two substantially planar end walls, which are substantially parallel to the first flow direction and which are arranged on respective sides of and substantially orthogonally to the guide walls, wherein each end wall of at least one of the flow channels exhibits an opening that communicates with the flow channel for removing at least part of the air, which has been released from the drainage water in the flow channel by said interaction, from the first chamber part.
Many prior art solutions present certain challenges, particularly for modernized systems. FIGS. 1A and 1B illustrate prior art. FIG. 1A illustrates certain aspects of the deaeration unit disclosed in U.S. Pat. No. 8,784,538. A guiding portion 2 directs drainage water flow in the machine cross direction out of the forming section, and comprises a plurality of substantially parallel, curved deflectors 4, which direct the drainage water to an outlet 34 of guiding portion 2.
A deaeration unit 3 has its extension substantially in the cross direction of the paper machine. The deaeration unit 3 includes a plurality of dividing guide walls 10 of sheet metal and roof portion 23, which exhibit a curved or bent shape. Each guide wall 10, 23 exhibits a free upstream end 11 that is arranged at the inlet 8 (which receives drainage water from the guiding portion 2 via outlet 34).
FIG. 1B illustrates detail of the interface between the outlet 34 of the guiding portion 2 and the inlet 8 of the deaeration unit 3. At this interface, the trailing edges of the curved deflectors 4 are oriented across the free upstream ends 11 of the guide walls 10 in the deaeration unit 3. This orientation causes flow streams in the channels exiting the guiding portion 2 to be “bisected” or “chopped” by the free upstream ends 11 of guidewalls 10. Such chopping increases turbulence and mixing as the spray “bounces off” these surfaces (e.g., edges between the guiding portion 2 into the de-aeration unit 3). Additionally, a single channel in the guiding portion ejects water and air into a plurality of channels in the deaeration unit, and a single channel in the deaeration unit receives water and from a plurality of channels in the guiding portion. This “criss-cross” orientation provides for fluidic communication among channels (e.g., between channels in the guiding portion, 2 between channels in the de-aeration unit 3, and their combinations), as shown by the schematic arrows in FIG. 1B. With separate channels in fluidic communication, air pressure throughout the channels is expected to be substantially equal.
A paper machine may include a turbine, for example as described in U.S. Pat. No. 6,398,913 (also published as US 2001/0018958 A1). A turbine disposed in the flow of whitewater after it has passed through the forming wire may recover energy from the whitewater (e.g., to generate electricity with a generator powered by the turbine).
Many forming sections generate a “mist” of fine droplets of liquid. Typically, these droplets decelerate quickly, and spread around the forming section (and even throughout the room) via convection of air currents. The mist may deposit on surfaces (e.g., floors, steps) and render them unsafe. A mist may comprise residual fibers. A mist may deposit as a “slime” or otherwise form a slippery surface. A deposited mist may clog or otherwise degrade various surfaces (e.g., parts of the machine). A mist may prevent a user or inspector from efficiently seeing various parts of the machine. In some cases, a mist can create a sheet break (a costly manufacturing defect). It is typically desirable to minimize mist formation and/or minimize the deleterious effects associated with mist (e.g., deposition, slime, corrosion).
U.S. Pat. No. 3,801,435 teaches a “paper machine saveall with de-aerator.” (Title) U.S. Pat. No. 3,960,653 teaches a “downflow control system for web making machines.” (Title) German patent document no. DE 199 38 799 A1 teaches “Die folgenden Angaben sind den vom Anmelder eingereichten Unterlagen entnommen.”